1. Field of the Invention
The present invention relates to magnetic amorphous alloys and to a method of annealing these alloys in a magnetic field. The present invention is also directed to amorphous magnetostrictive alloys for use in a magnetomechanical electronic article surveillance system. The present invention furthermore is directed to a magnetomechanical electronic article surveillance system employing such a marker, as well as to a method for making the amorphous magnetostrictive alloy and a method for making the marker.
2. Description of the Prior Art
It is well known from Chikazumi, Physics of Magnetism (Robert E. Krieger Publishing Company, Malbar, Fla.) chapter 17, p. 359 ff. (1964), for example, that most ferromagnetic alloys exhibit a uniaxial anisotropy when they are heat-treated in a magnetic field whereby the induced magnetic easy axis is parallel to the direction of the annealing field or, more generally, parallel to the domain magnetization during annealing. The aforementioned Chikazumi text gives an example for the magnetization curve of a permalloy (crystalline Fexe2x80x94Ni alloy) sample measured in a direction perpendicular to the induced magnetic easy axis. Chikazumi notes that in this case the magnetization takes place through a rotation of each magnetic domain giving rise to a linearly ascending magnetization curve.
Luborsky et al., xe2x80x9cMagnetic Annealing of Amorphous Alloysxe2x80x9d, IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975) give an early example for magnetic field annealing of amorphous alloys. They transversely field-annealed amorphous Fe40Ni40P14B6 alloy strips in a magnetic field of 4 kOe which was oriented across the ribbon width, i.e. perpendicular to the ribbon axis and in the ribbon plane. After a 2 hrs. treatment at 325xc2x0 C. and subsequent cooling of 50 deg/min and 0.1 deg/min, for example, they found a hysteresis loop with virtually vanishing remanence and linear dependence of the magnetization versus the applied field up to ferromagnetic saturation which occurs when the applied field equals or exceeds the induced anisotropy field. The authors attributed their observation to the fact that the magnetic field annealing induces a magnetic easy axis transverse to the ribbon direction and that upon applying a magnetic field the magnetization changes by rotation out of this easy axis.
Actually amorphous metals are particularly sensitive to magnetic field annealing owing to the absence of magneto-crystalline anisotropy as a consequence of their glassy non-periodic structure. Amorphous metals can be prepared in the form of thin ribbons by rapidly quenching from the melt which allows a wide range of compositions. Alloys for practical use are basically composed of Fe, Co and/or Ni with an addition of about 15-30 at % of Si and B (Ohnuma et al., xe2x80x9cLow Coercivity and Zero Magnetostriction of Amorphous Fexe2x80x94Coxe2x80x94Ni System Alloysxe2x80x9d Phys. Status Solidi (a) vol. 44, pp. K151 (1977)) which is necessary for glass formation. The virtually unlimited miscibility of the transition metals in the amorphous state yields a large versatility of magnetic properties. According to Luborsky et al., xe2x80x9cMagnetic Anneal Anisotropy in Amorphous Alloysxe2x80x9d, IEEE Trans. on Magnetics MAG-13, p. 953-956 (1977) and Fujimori xe2x80x9cMagnetic Anisotropyxe2x80x9din F. E. Luborsky (ed) Amorphous Metallic Alloys, Butterworths, London, pp. 300-316 (1983) alloy compositions with more than one metal species are particularly susceptible to the magnetic field anneal treatment. Thus, the magnitude of the induced anisotropy Ku can be varied by choice of the alloy composition as well as by appropriate choice of the annealing temperature and time to range from a few J/m3 up to about 1 kJ/m3. Accordingly the anisotropy field which is given by HK=2 Ku/Js (cf. Luborsky et al., xe2x80x9cMagnetic Annealing of Amorphous Alloysxe2x80x9d, IEEE Trans. on Magnetics MAG-11, p. 1644-1649 (1975); Js is the saturation magnetization) and which, for a transversely field-annealed material, defines the field up to which the magnetization varies linearly with the applied field before reaching saturation, can be varied from values well below 1 Oe up to values of approximately HK≈25 Oe.
The linear characteristics of the hysteresis loop and the low eddy current losses both associated with transversely field-annealed amorphous alloys are useful in a variety of applications such as transformer cores, for example (cf. Herzer et al, xe2x80x9cRecent Developments in Soft Magnetic Materialsxe2x80x9d, Physica Scripta vol T24, p 22-28 (1988)). Another field of application where transversely annealed amorphous alloys are particularly useful makes use of their magnetoelastic properties which is described in more detail in the following.
Becker et al., Ferromagnetismus (Springer, Berlin), ch. 5, pp. 336 (1939) or Bozorth, Ferromagnetism (d. van Nostrand Company, Princeton, N.J.) ch. 13, p 684 ff (1951) explain in their textbooks that the magnetostriction associated with rotation of the magnetization vector is responsible for the fact that in ferromagnetic materials Young""s modulus changes with the applied magnetic field, which is usually referred to as the xcex94E effect.
Consequently U.S. Pat. No. 3,820,040 and Berry et al. xe2x80x9cMagnetic annealing and Directional Ordering of an Amorphous Ferromagnetic Alloyxe2x80x9d, Physical Reviews Letters, vol. 34, p. 1022-1025 (1975) realized that an amorphous Fe-based alloy, when transversely field annealed, exhibits a xcex94E effect two orders of magnitude larger than for crystalline iron. They attributed this striking difference to the lack of magnetocrystalline anisotropy in the amorphous alloy, which allows a much greater response to the applied stress by magnetization rotation. They also demonstrated that [a] annealing in a longitudinal field largely suppresses the xcex94E effect since in this condition the domain orientations are not susceptible to stress-induced rotation. In the Berry et al. 1975 article it is recognized that the enhanced xcex94E effect in amorphous metals provides a useful means to achieve control of the vibrational frequency of an electromechanical oscillator with the help of an applied magnetic field.
The possibility to control the vibrational frequency by an applied magnetic field was found to be particularly useful in European Application 0 093 281 for markers for use in electronic article surveillance (EAS). The magnetic field for this purpose is produced by a magnetized ferromagnetic strip (bias magnet) disposed adjacent to the magnetoelastic resonator, with the strip and resonator being contained in a marker or tag housing. The change in effective magnetic permeability of the marker at the resonance frequency provides the marker with signal identity. This signal identity can be removed by changing the resonant frequency by means of the applied field. Thus the marker can, for example, be deactivated by degaussing the bias magnet, which removes the applied magnetic field, and thus changes the resonant frequency appreciably. Such systems originally (cf. European Application 0 093 281, and Application PCT WO 90/03652) used markers made of amorphous ribbons in the xe2x80x9cas preparedxe2x80x9d state which also can exhibit an appreciable xcex94E effect owing to uniaxial anisotropies associated with production-inherent mechanical stresses.
U.S. Pat. No. 5,469,140 discloses that the application of transverse field annealed amorphous magnetomechanical elements in electronic article surveillance systems removes a number of deficiencies associated with the markers of the prior art which use as prepared amorphous material. In an example, this patent describes a linear behavior of the hysteresis loop up to an applied field of at least about 10 Oe. This linear behavior associated with the transverse field annealing avoids the generation of harmonics which can produce undesirable alarms in other types of EAS systems (i.e., harmonic systems). Such interference with harmonic systems actually is a severe problem with the aforementioned magneto-elastic markers of the prior art, due the non-linear hysteresis loop typical associated with the as prepared state of amorphous alloys, since it is this non-linear behavior which (undesirably) triggers an alarm in a harmonic EAS system. This patent further teaches that heat treatment in a magnetic field significantly improves the consistency in terms of the resonant frequency of the magnetostrictive strips. A further advantage of such annealed resonators is their higher resonant amplitude. This patent also teaches that a preferred material is an Fexe2x80x94Co alloy which contains at least about 30 at % Co, whereas earlier materials of the prior art such as Fe40Ni38Mo3B18, disclosed in the aforementioned PCT Application WO 90/03652 are unsuitable in pulse-field magnetomechanical EAS systems since annealing such materials undesirably reduces the ring down period of the signal. In German Gebrauchsmuster G 94 12 456.6 the present inventor recognized that a long ring-down time can be achieved by choosing an alloy composition which reveals a relatively high induced magnetic anisotropy and that, therefore, such alloys are particularly suited for magnetoelastic markers in article surveillance systems. Herzer teaches that the desired high ring-down times can be also achieved at lower Co-contents down to about 12 at % if, starting from a Fexe2x80x94Co-based alloy, up to about 50% of the Fe and/or Co is substituted by Ni. The need for a linear loop with relatively high anisotropy and the benefit of alloying Ni in order to reduce the Co-content for such magnetoelastic markers was later on reconfirmed by the disclosure of U.S. Pat. No. 5,628,840.
The field annealing in the aforementioned examples was done across the ribbon width i.e. the magnetic field direction was oriented perpendicular to the ribbon axis and in the plane of the ribbon surface. This technique will be referred to herein, and is known in the art, as transverse field-annealing. The strength of the magnetic field has to be strong enough in order to saturate the entire ribbon ferromagnetically across the ribbon width. This can be achieved in magnetic fields as low as a few hundred Oe. Such transverse field-annealing can be performed, for example, batch-wise either on toroidally wound cores or on pre-cut straight ribbon strips. Alternatively, and as disclosed in detail in U.S. Pat. No. 5,469,140, the annealing can be performed in a continuous mode by transporting the alloy ribbon from one reel to another reel through an oven in which a transverse saturating field is applied to the ribbon.
The change of magnetization by rotation and the associated magnetoelastic properties are primarily related to the fact that there is a uniaxial anisotropy axis perpendicular to the applied operational magnetic field. The anisotropy axis need not necessarily be in the ribbon plane like in the case of the transversely field annealed samples; the uniaxial anisotropy can also be caused by mechanisms other than field annealing. A typical situation is, for example, that the anisotropy is perpendicular to the ribbon plane. Such an anisotropy can arise again from magnetic field annealing but this time in a strong field oriented normal to the ribbon""s plane, as taught by Gyorgy, in Metallic Glasses, 1978, Proc. ASM Seminar Sept. 1976 (American Society for Metals, Metals Park, Ohio) ch. 11, pp 275-303, U.S. Pat. No. 4,268,325, Grimm et al., 1985, xe2x80x9cMinimization of Eddy Current Losses in Metallic Glasses by Magnetic Field Heat Treatmentxe2x80x9d, Proceedings of the SMM 7 conference in Blackpool (Wolfson Centre for Magnetics Technology, Cardiff) p. 332-336, de Wit et al., 1985 xe2x80x9cDomain patterns and high-frequency magnetic properties of amorphous metal ribbonsxe2x80x9d J. Appl. Phys. vol 57, pp. 3560-3562 (1985), and Livingston et al., xe2x80x9cMagnetic Domains in Amorphous Metal ribbonsxe2x80x9d, J. Appl. Phys. vol. 57, pp 3555-3559 (1985), which hereafter will be referred to as perpendicular field-annealing. Other sources of such a perpendicular anisotropy can arise from the magnetostrictive coupling with internal mechanical stresses associated with the production process (see the aforementioned Livingston et al., xe2x80x9cMagnetic Domains in Amorphous Metal ribbonsxe2x80x9d article and the aforementioned chapter by Fujimori in F. E. Luborsky (ed)) or e.g. induced by partial crystallization of the surface (Herzer G. xe2x80x9cSurface Crystallization and Magnetic Properties in Amorphous Iron Rich Alloysxe2x80x9d, J. Magn. Magn. Mat., vol. 62, p. 143-151 (1986)).
When the magnetic easy axis is perpendicular to the ribbon plane, the large demagnetization factor requires very fine domain structures in order to reduce magnetostatic stray field energy (cf. Landau et al. in Electrodynamics of Continuous Media, Pergamon, Oxford, England, ch 7. (1981)). Domain widths observed are typically 10 xcexcm or less and the visible domains are generally closure domains while ribbons with an anisotropy across the ribbon width exhibit wide transverse slab domains, typically about 100 xcexcm in width (as taught by the aforementioned Gyorgy article and the aforementioned de Wit et al. article, and Mermelstein, xe2x80x9cA Magnetoelastic Metallic Glass Low-Frequency Magnetometerxe2x80x9d, IEEE Transactions on Magnetics, vol. 28, p. 36-56 (1992)).
One of the first examples for perpendicular field annealing was given in the aforementioned article by Gyorgy in which, for a Co-based amorphous alloy, the domain structure after said annealing treatment is compared with that obtained after a transverse field-anneal treatment and a longitudinal field anneal treatment, respectively. Gyorgy states that the domain structure of the perpendicularly annealed sample is typical for a uniaxial material with the easy axis normal to the surface.
The latter finding was confirmed in the aforementioned de Wit et al. article wherein two samples of a near-zero magnetostrictive amorphous Co-base alloy are compared, one having been transversely field-annealed in a field of 0.9 kOe and the other having been perpendicularly field annealed in a field of 15 kOe. de Wit et al. found that, as already mentioned above, in both cases the magnetization process is controlled by rotation which results in an essentially linear behavior of the magnetization with the applied field. The aforementioned Mermelstein article reaches a similar conclusion for a highly magnetostrictive amorphous Fe-based ribbon which was transversely and perpendicularly field-annealed, respectively, in a magnetic field of 8.8 kOe. Mermelstein posits that in both cases the magnetization process is controlled by rotation of the magnetization vector towards the applied field, and thus concludes it is sufficient to use a single model in order to describe the magnetic and magnetoelastic properties as well as the effect of eddy currents in both cases. Mermelstein""s investigations were directed to a magnetoelastic field sensor using these samples and he concludes that both types of domain structures exhibit nominally equivalent noise baselines and that any differences in the sensor""s sensitivity are only to be attributed to the differing anisotropy fields associated with dissimilarities in the heat treatment.
Still, as noted above de Wit et al. found that although essentially linear, the hysteresis loop of the perpendicularly annealed sample revealed a non-linear opening in its center region which is accompanied by enhanced eddy current losses, unlike the transversely annealed sample. This finding has been confirmed in the aforementioned Grimm et al., article which reports investigation of the perpendicular anisotropy in amorphous FeCo- and FeNi-based alloys induced by annealing in a magnetic field of 9 kOe oriented normal to the ribbon surface. Grimm et al. attribute this non-linearity to switching processes in the closure domains. Only in the case of the sample which had the highest magnetostriction (xcexs≈22 ppm) did they find a substantially linear magnetization loop with negligible hysteresis and considerably reduced eddy current losses. They found that in this case magnetostrictive interactions favor the closure domains to be oriented perpendicular to the applied field, which results in a less complex magnetization process within the closure domains. In contrast, the closure domain stripes are oriented parallel to the applied field for samples with lower magnetostriction constants (i.e. about 9 ppm in one example or a near-zero magnetostrictive sample), which results in the aforementioned non-linearity in the hysteresis loop""s center region.
Comparable results also have been disclosed in the aforementioned U.S. Pat. No. 4,268,325 which describes annealed ring-laminated, toroidal cores assembled from punchouts from a 2 cm wide amorphous glassy Fe40Ni40B20 ribbon in a perpendicular field of 2 kOe and a circumferential field of 1 Oe. According to this patent, the application of such a perpendicular field during annealing results in a sheet having an easy magnetic axis essentially normal to the sheet plane. The result was a relatively linear magnetization loop but again with a non-linear opening in its center region and enhanced AC losses. The aforementioned U.S. Pat. No. 4,268,325, moreover, teaches that it is advantageous to apply in a second annealing step a magnetic field normal to the direction of the first field in order to minimize AC hysteresis losses. Indeed the losses of the cited sample could be improved by subsequent annealings in a circumferential field. This second annealing step increases the remanence, and thus the non-linearity, and led to a minimum at an enhanced remanence of about 3.5 kG where the hysteresis loop was substantially non-linear.
All these observations teach that no real benefit is associated with perpendicular field-annealing over transverse field-annealing. Indeed, transverse field-annealing seems to be clearly advantageous if a linear hysteresis loop and low eddy current losses are required for whatever application. Moreover, transverse field-annealing is much easier to conduct experimentally than perpendicular field-annealing due in part to the field strengths needed to saturate the ribbon ferromagnetically in the respective cases in order to obtain a uniform anisotropy. Owing to their magnetic softness, amorphous ribbons can be generally saturated ferromagnetically in internal magnetic fields of a few hundred Oersteds. The internal magnetic field in a sample with finite dimensions, however, is composed of the externally applied field and the demagnetizing field, which acts opposite to the applied field. While the demagnetizing field across the ribbon width is relatively small, the demagnetizing field normal to the ribbon plane is fairly large and, for a single ribbon, almost equals the component of the saturation magnetization normal to the ribbon plane. Accordingly, in the aforementioned U.S. Pat. No 4,268,325 it is taught that the strength of the perpendicularly applied magnetic field preferably should be at least about 1.1 times the saturation induction at the annealing temperature. This is typically accomplished by a field strength of about 10 kOe or more as reported in the aforementioned papers relating to perpendicular field annealing. In comparison transverse field-annealing can be successfully done in considerably lower fields in excess of a few hundred Oe only. The aforementioned U.S. Pat. No. 5,469,140 as well as European Application 0 737 986, for example, teach that for transverse field-annealing a field strength in excess of 500 Oe or 800 Oe is enough to achieve saturation. Of course such a moderate field can be realized in a much easier and a more economic way than the high fields necessary for perpendicular annealing. Thus, lower magnetic fields allow a wider gap in the magnet, which facilitates the construction of the oven which has to be placed within this gap. If the field is produced by an electromagnet, moreover, the power consumption is reduced. For a yoke built of permanent magnets lower field strengths can be realized with less and/or cheaper magnets.
According to the state of the prior art discussed above, the transverse field-annealing method seems to be much more preferable over the perpendicular field-annealing method for a variety of reasons. The present inventor has recognized, however, that an annealing method in which the magnetic field applied during annealing has a substantial component out of the ribbon plane may, if properly performed, yield much better magnetic and magneto-elastic properties than the conventional methods taught by the prior art.
It is an object of the present invention to provide a method of reducing the eddy current losses of a ferromagnetic ribbon which in operation is magnetized by a static magnetic bias field.
More specifically it is an object of the present invention to provide a magnetostrictive alloy, and a method for annealing same, in order to produce a resonator having properties suitable for use in a magnetomechanical electronic surveillance system with better performance than conventional resonators.
It is another objective of this invention to provide such a magnetostrictive amorphous metal alloy for incorporation in a marker in a magnetomechanical surveillance system which can be cut into an oblong, ductile, magnetostrictive strip which can be activated and deactivated by applying or removing a pre-magnetization field H and which, in the activated condition can be excited by an alternating magnetic field so as to exhibit longitudinal, mechanical resonance oscillations at a resonant frequency fr which after excitation are of high signal amplitude.
It is a further object of this invention to provide such an alloy wherein only a slight change in the resonant frequency fr occurs given a change in the magnetization field strength.
A further object is to provide such an alloy wherein the resonant frequency fr changes significantly when the marker resonator is switched from an activated condition to a deactivated condition.
Another object of the present invention is to provide such an alloy which, when incorporated in a marker for a magnetomechanical surveillance system, does not trigger an alarm in a harmonic surveillance system.
It is also an object of this invention to provide a marker embodying such a resonator, and a method for making a marker, suitable for use in a magnetomechanical surveillance system.
Another object of this invention is to provide a magnetomechanical electronic article surveillance system which is operable with a marker having a resonator composed of such an amorphous magnetostrictive alloy.
The above objects are achieved in a resonator, a marker embodying such a resonator and a magnetomechanical article surveillance system employing such a marker, wherein the resonator is an amorphous magnetostrictive alloy and wherein the raw amorphous magnetostrictive alloy ribbon is annealed in a such a way that a fine domain structure is formed with a domain width less than about 40 xcexccm or 1.5 times the thickness of the ribbon, and that an anisotropy is induced which is perpendicular to the ribbon axis and points out of the ribbon plane at an angle larger than 50 up to 90xc2x0 with respect to the ribbon plane. The lower bound for the anisotropy angle is necessary to achieve the desired refinement of the domain structure which is necessary to reduce eddy current losses, and thus improves the signal amplitude, and hence improves the performance of the electronic article surveillance system using such a marker.
This can be accomplished, for example, in an embodiment of the invention wherein crystallinity is introduced from the top and bottom surfaces of the ribbon or strip to depth of about 10% of the strip or ribbon thickness at each surface, which results in an anisotropy perpendicular to the ribbon axis and perpendicular to the ribbon plane. Thus, as used herein, xe2x80x9camorphousxe2x80x9d (when referring to the resonator) means a minimum of about 80% amorphous (when the resonator is viewed in a cross-section). In another embodiment a saturating magnetic field is applied perpendicular to the ribbon plane such that the magnetization is aligned parallel to that field during annealing. Both treatments result in a fine domain structure, an anisotropy perpendicular to the ribbon plane and a substantially linear hysteresis loop. As used herein xe2x80x9csubstantially linearxe2x80x9dincludes the possibility of the hysteresis loop still exhibiting a small non-linear opening in its center. Although such a slightly non-linear loop triggers fewer false alarms in harmonic systems compared to conventional markers, it is desirable to virtually remove the remaining non-linearity.
Therefore, the annealing is preferably done in such a way that the induced anisotropy axis is at an angle less than 90xc2x0 with respect to the ribbon plane, which yields an almost perfectly linear loop. Such an xe2x80x9cobliquexe2x80x9d anisotropy can be realized when the magnetic annealing field has an additional component across the ribbon width.
Thus the above objects can be achieved preferably by annealing the amorphous ferromagnetic metal alloy in a magnetic field of at least about 1000 Oe oriented at an angle with respect to the ribbon plane such that the magnetic field has one significant component perpendicular to the ribbon plane, one component of at least about 20 Oe across the ribbon width and a nominally negligible component along the ribbon axis to induce a magnetic easy axis which is oriented perpendicular to the ribbon axis but with a component out of the ribbon plane.
The oblique magnetic easy axis can be obtained, for example, by annealing in a magnetic field having a field strength which is sufficiently high so as to be capable of orienting the magnetization along its direction and at an angle between about 100 and 80xc2x0 with respect to a line across the ribbon width. This, however requires very high field strengths of typically around 10 kOe or considerably more, which are difficult and costly in realization.
A preferred method in order to achieve the above objects therefore includes applying a magnetic annealing field whose strength (in Oe) is lower than the saturation induction (in Gauss) of the amorphous alloy at the annealing temperature. This field, typically 2 kOe to 3 kOe in strength, is applied at angle between about 60xc2x0 and 89xc2x0 with respect to a line across the ribbon width. This field induces a magnetic easy anisotropy axis which is parallel to the magnetization direction during annealing (which typically does not coincide with the field direction for such moderate field strengths) and which is finally oriented at angle of at least about 5-10xc2x0 out of the ribbon plane and, at the same time, perpendicular to the ribbon axis.
Apart from its direction, the aforementioned oblique anisotropy is independently characterized by its magnitude which is in turn characterized by the anisotropy field strength Hk. As described earlier the direction is primarily set by the orientation and strength of the magnetic field during annealing. The anisotropy field strength (magnitude) is set by a combination of the annealing temperature-time profile and the alloy composition, with the order of anisotropy magnitude being primarily varied (adjusted) by the alloy composition with changes from an average (nominal) magnitude then being achievable within about +/xe2x88x9240% of the nominal value by varying (adjusting) the annealing temperature and/or time.
A generalized formula for the alloy composition which, when annealed as described above, produces a resonator having suitable properties for use in a marker in an electronic magnetomechanical article surveillance or identification system, is as follows,
FeaCobNicSixByMz 
wherein a, b , c, y, x, and z are in at %, wherein M is one or more glass formation promoting element such as C, P, Ge, Nb, Ta and/or Mo and/or one or more transition metals such as Cr and/or Mn and wherein
15 less than a less than 75
0 less than b less than 40
0xe2x89xa6c less than 50
15 less than x+y+z less than 25
0xe2x89xa6z less than 4
so that
a+b+c+x+y+z=100.
The detailed composition has to be adjusted to the individual requirements of the surveillance system. Particularly suited compositions generally reveal a saturation magnetization Js at the annealing temperature which is preferably less then about 1 T (=10 kG) and/or a Curie temperature Tc ranging from about 350xc2x0 C. to about 450xc2x0 C. Given these limits, more appropriate Fe, Co and Ni contents can be selected e.g. from the data given by Ohnuma et al., xe2x80x9cLow Coercivity and Zero Magnetostriction of Amorphous Fe=13 Coxe2x80x94Ni System Alloysxe2x80x9d Phys. Status Solidi (a) vol. 44, pp. K151 (1977). In doing so one should have in mind that, Js and Tc can be decreased or increased by increasing or decreasing the sum of x+y+z, respectively. Preferably, those compositions should be generally selected which, moreover, when annealed in a magnetic field, have an anisotropy field of less than about 13 Oe.
For one major electronic article surveillance system on the market, the desired objects of the inventions can be realized in a particularly advantageous way by applying the following ranges to the above formula
15 less than a less than 30
10 less than b less than 30
20 less than c less than 50
15 less than x+y+z  less than 25
0xe2x89xa6z less than 4
and even more preferably
15 less than a less than 27
10 less than b less than 20
30 less than c less than 50
15 less than x+y+z less than 20
0 less than x less than 6
10 less than y less than 20
0xe2x89xa6z less than 3
Examples of such particularly suited alloys for this EAS system have e.g. a composition such as Fe24Co18Ni40Si2B16, Fe24Co16Ni43Si1B16 or Fe23Co15Ni45Si1B16, a saturation magnetostriction between about 5 ppm and about 15 ppm, and/or when annealed as described above have an anisotropy field of about 8 to 12 Oe. These examples in particular exhibit only a relatively slight change in the resonant frequency fr given a change in the magnetization field strength i.e. |df/dH| less than 700 Hz/Oe but at the same time the resonant frequency fr changes significantly by at least about 1.4 kHz when the marker resonator is switched from an activated condition to a deactivated condition. In a preferred embodiment such a resonator ribbon has a thickness less than about 30 xcexcm, a length of about 35 mm to 40 mm and a width less then about 13 mm preferably between about 4 mm to 8 mm i.e., for example, 6 mm.
Other applications such as electronic identification systems or magnetic field sensor rather require a high sensitivity of the resonant frequency to the bias field i.e. in such case a high value of |df/dHI greater than 1000 Hz/Oe is required. Examples of particularly suited compositions for this case have e.g. a composition such as Fe62Ni20Si2B16, Fe40Co2Ni40Si5B13, Fe37Co5Ni40Si2B16or Fe32Co10Ni40Si1B16, a saturation magnetostriction larger than about 15 ppm and/or when annealed as described above have an anisotropy field ranging from about 2 Oe to about 8 Oe.
Additionally, the reduction of eddy current losses by means of the heat treatment described herein can be of benefit for non-magneto-elastic applications and can enhance the performance of a near-zero magnetostrictive Co-based alloy when used e.g. in toroidally wound cores operated with a pre-magnetization generated by a DC current.